Heretofore, as a separator to be disposed between positive and negative electrodes of an electric double layer capacitor, a polyethylene non-woven fabric, a polypropylene non-woven fabric, a polyester non-woven fabric, kraft paper, a rayon/sisal mixed sheet, a manila hemp sheet or a glass fiber sheet is, for example, known (e.g. JP-A-9-45586, JP-A-1-304719). The role of a separator is to electrically insulate the positive electrode from the negative electrode on one hand and to facilitate transfer of ions in the electrolyte, which takes place accompanying charge and discharge, on the other hand.
In recent years, an attention has been drawn to an electric double layer capacitor for high power applications. However, with a separator made of organic fibers of e.g. a polyethylene, the ion conductivity is low, and the internal resistance of the electric double layer capacitor is high, since the electrolyte absorbing property and the electrolyte-holding property are low. Accordingly, it instantaneous large current discharge was carried out as one of main characteristics of an electric double layer capacitor, the voltage drop was substantial, such being not practical.
Further, a conventional separator made of paper is excellent in heat resistance and tensile strength and is sometimes effective for an electric double layer capacitor to be used for a power source wherein no large current discharge is carried out, like a hybrid power source used with a solar cell. However, when a conventional separator made of paper is used for an electric double layer capacitor for high power applications, wherein the electrolyte is non-aqueous, the ion permeability tends to be inadequate.
With an electric double layer capacitor for high power applications, it is desired to lower the resistance and to increase the capacitance per unit volume (hereinafter referred to as capacitance density), and it is accordingly required to make the separator as thin as possible. At the same time, the separator is required to have a high porosity from the viewpoint of the electrolyte absorbing property and the electrolyte-holding property. However, if a separator made of paper having a high porosity is made thin, the insulating property between the positive and negative electrodes tends to be inadequate, thus leading to micro-short circuiting, self-discharge or a decrease in the yield in the production of the capacitor.
Further, when an electrolyte is impregnated to an element formed by laminating a plurality of electrodes with separators interposed or winding two sheets of electrodes with a separator interposed, so that the electrodes and the separator are laminated in close contact to one another, for example, with a separator made of a paper having a thickness of not more than 100 μm, there is a limit in increasing the porosity, and it takes much time to inject the electrolyte, thus leading to a problem in the productivity.
Accordingly, it is an object of the present invention to provide an electric double layer capacitor having a low resistance and a high capacitance density with good productivity by employing a separator which is excellent in heat resistance, ion permeability, electrolyte-absorbing property and electrolyte-holding property and which has high strength even when it is thin, and is excellent in the electrical insulating property.